Increased demands placed on electrical performance of connectors in high speed applications, including optical transceiver and high data rate rigid disk drive storage products, require very short connection lengths to minimize interconnection inductances, contact resistances, and signal lengths. Use of short interconnects also enables greater control for impedance matching at carrier interconnection interfaces as well. A desirable interconnection scheme possessing these attributes is best provided through direct mating between carrier contact pads, thus eliminating the need for secondary connector components such as headers and receptacles. Elimination of secondary connector hardware also allows for added flexibility within constrained three dimensional form factor packaging spaces commonly encountered within rigid disk drives, printers, and a variety of consumer electronics hardware. These interconnection designs are commonly referred to as pad on pad connections.
For flex circuit pad on pad contacts, an interconnection scheme is usually provided with raised metal bumps on flex contact pad locations. The raised cable contacts make electrical interconnections with opposing pads on carrier electronics and provide reliability when contacts possess noble or semi-noble surface finishes and are mechanically affixed with secondary hardware that provides necessary normal forces to the mated pad on pad contacts. Raised contact bumps on flex circuits are usually made in one of two ways. The first method typically involves hot deformation of a flex circuit at the base metal copper contact pad locations. After deformation, the raised contacts must be rigidized to prevent base metal copper bump contact relaxation and creep in the presence of constant loads required for interconnection reliability (loads are typically 70-300 gmF per contact). Rigidization of the thin, soft raised copper bumps is accomplished by plating with a thick (10-30 .mu.m) coating of nickel, followed by a final overplating of tin, palladium, gold, or hardened contact finish such as palladium-nickel, nickel or cobalt doped gold. The second method used for raised bump formation does not require hot deformation to define the raised contact bumps. In this case, bumps are made by selective application of very thick plated coatings of copper and overplate layers at the contact sites, as in the first method. Regardless of the process used, the contacts must be sufficiently resistant to relaxation and wear prompted from multiple plugging, merges, and sustained connector loads. Unfortunately, manufacture of both types of bump contact structures is very expensive, as selective platings and high plating thicknesses require multiple and slow process sequences. Reliability of these structures can also be suspect, since use of thick nickel coatings can result in residual stress buildup in both the plating and base metal and prompt cracking of bumps and adjoining traces in process and/or in field service. In addition, hot deformed and plated bump structures are not sufficiently rigid to survive additional lamination cycles without sustaining flattening damage if required on certain flex carrier designs.